Automated optical inspection and sorting systems have been used to inspect and sort various target specimens including fruits and vegetables, processed meats, baked goods, and other foodstuffs, to separate different types of recyclable material, and to sort foreign or defective items from supplies of wood chips. These systems typically employ video cameras with charge-coupled device line scan cameras to acquire images of target specimens moved on a conveyor belt across an optical scanning area. Illumination of the specimens is generally provided by broad-spectrum fluorescent lamps. Signal processing circuitry identifies variations in the shade of target specimen images and sorts target specimens accordingly.
Inspection and sorting systems utilizing color images are well suited to inspecting and sorting specimens when the unacceptable and acceptable items are characterized by subtle differences in color, shade, and hue. Progress in detecting subtle variations in color has improved the precision and quality of color inspection and sorting systems. Such progress has also reduced the waste among specimens processed by such color inspection and sorting systems. These recent improvements in color sorting precision have not always also resulted in increased sorting speed.
Another area of ongoing development in optical inspection and sorting system is directed toward increasing the processing efficiency, that is, the quantity of target specimens inspected and sorted per unit time. Advances in signal processing techniques provide some increases in the speed at which images of the specimens can be acquired and processed. However, increased image acquisition speeds result in diminished scanning time for each individual target specimen and, consequently, a reduction in the amount of light received by photoreceptors from the target specimens. Illumination of target specimens thus becomes important.
Light intensity at a target specimen varies as the inverse square of the distance from the light source. Inspection and sorting systems having video cameras and light sources positioned at a distance from the target specimens, for example, to view a sufficiently large surface area, generally suffer a substantial decrease in the intensity of light received from the inspection area and target specimens. Increases in image acquisition speeds and video camera and light source position therefore must be carefully coordinated to improve the performance of inspection and sorting systems.
Optical defect inspection systems typically employ conventional tubular fluorescent lamps to illuminate the specimen inspection field. Such conventional fluorescent lamps include, for example, Sylvania cool-white VHO fluorescent lamp model F 72 T 12 (212 watts) available from GTE Electrical Products, One Stamford Forum, Stamford, Conn. 06904 and similar cool-white VHO fluorescent lamps available from North American Phillips, Phillips Lighting Products, 200 Franklin Sq. Dr., Sommerset, N.J. 08875. Conventional tubular fluorescent lamps emit a broad spectrum of radiation and typically comprise a large-diameter elongated tube with a layer of phosphorous material coating an inner surface. Conventional fluorescent light sources are inefficient in the context of optical defect and inspection systems because they waste unacceptably large quantities of their radiated light energy. The unacceptable losses occur for several reasons.
Much of the radiation emitted from conventional fluorescent lamp sources in optical inspection systems is absorbed by the conveyor belt and portions of the target specimens that are not visible to the image acquisition sensors of the video camera. Absorbed radiation can shorten the useful life of the conveyor belt and the shelf or storage life of the target specimen. Furthermore, much of the broad-spectrum radiation emitted by conventional fluorescent sources that is reflected by the surface of the target specimens does not contribute to the sorting decision. That is, much of the reflected light contributes little information useful for inspection and sorting decisions. At the same time, conventional broad-spectrum sources often provide insufficient radiation of those wavelengths that do contribute to distinguishing the features upon which sorting decisions are based. As a result of limitations in illumination of target specimens, automated inspection and sorting equipment employing conventional fluorescent lamps is often unable to keep pace with the increased image acquisition and processing capabilities of current and emerging technologies.
A phosphor or other coating on the inner surface of the tubular glass envelope of a conventional fluorescent lamp is adapted to emit visible light upon absorption of ultraviolet radiation that is produced when the lamp electrodes, which typically contain mercury or other emissive material, are ionized by application of an electrical current. The phosphor coating on the inner surface of the lamp envelope fluoresces and reemits a substantial portion of the ultraviolet radiation as visible light. The spectral characteristics of the visible light are determined principally by the composition of the fluorescent powders used for the phosphor coating. In inspection systems employing conventional fluorescent lamps as the only source of illumination, relatively few of the wavelengths of visible light emitted by the fluorescent lamp are used for the inspection and sorting decisions. In some such systems, bandfilter or other color-selective filters are used to cull the useful wavelengths from the many wavelengths emitted.
Conventional fluorescent lamps also have the disadvantage that luminous output degenerates over time and is temperature dependent. Moreover, conventional fluorescent lamps have a slow response time at start-up. The resulting inadequate and inconsistent lighting can significantly reduce the accuracy of inspection and sorting decisions.
Laser-based illumination systems are capable of directing high-intensity light at target specimens but are typically expensive and ineffective for inspection and sorting that requires multiple wavelengths of light. The range and variety of spectral output from currently available laser-based illumination systems is limited. The surface area that can be effectively illuminated by laser-based illumination systems also is limited.